How to make a 3D character move only left and right in 3D environment?

I am starting to create a platform third person causal game. The third person character from the download package isn’t helping. I don’t know what to do here. I want to make the character model move just left and right and jump. I need to know how to make the camera follow the character as the player moves up on screen?

This Script should work just apply to player!

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {

public float speed;

void Awake () {

}

void FixedUpdate () {

	if (Input.GetKey("a")) {
		rigidbody.AddForce (-Vector3.right * speed * Time.deltaTime);
	}
	if (Input.GetKey("d")) {
		rigidbody.AddForce (Vector3.right * speed * Time.deltaTime);
	}
}

This is also a C# script!